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(Leipe, 1989). The contractile vacuole system exits to the exterior through a pore supported by helically disposed microtubules (Rodrigues de Santa Rosa, 1976). In some Holophrya (formerly Prorodon ) species, there is a set of collecting canals extending anteriorly beneath the somatic cortex from the posterior contractile vacuole . These canals are associated with prekinetosomal micro- tubules , which may provide structural support (Hiller, 1993a). 13.4 Oral Structures As the class name suggests, the prostomes are cili- ates whose oral region is at the anterior end of the body, although it may be slightly subapical (Fig. 13.2). The oral opening is typically permanent and flanked by oral dikinetids that border one side of the oral region in Plagiocampa (Foissner, 1978) or surround the entire cytopharyngeal complex (Hiller, 1991, 1993b; Huttenlauch, 1987). Brosse polykinetids are situated on the opposite side to the oral dikinetids in Plagiocampa (Foissner, 1978), almost enclosed by them in some Coleps species (Wilbert & Schmall, 1976), or completely enclosed by them in Balanion species. The brosse kinetids of Balanion may be reduced to two kinetosomes with reduced ciliation (Bardele, 1999; Jakobsen & Montagnes, 1999). The brosse in other genera can be more extensive, ranging from one up to five rows of kinetosomes embedded in the somatic infraciliature. In Holophrya (formerly Prorodon ) species, Hiller and Bardele (1988) have identified two major patterns: the aklitoloph pattern in which the somatic kineties do not terminate laterally on the brosse rows and the enklitoloph pattern in which the somatic kineties terminate on the right of the brosse rows (i.e., dexiotrop ), on the left of the brosse rows (i.e., aristerotrop ), and on both sides of the brosse (i.e., syntrop ). They speculated that these patterns might indicate four genera of prorodontids . A brosse has also been identified in Cryptocaryon (Diggles, 1997). Early ultrastructural research on prostomes sug- gested that it was transverse microtubules arising from the oral dikinetids that surrounded the oral region (de Puytorac & Grain, 1972). Huttenlauch and Bardele (1987) demonstrated that these micro- tubules were in fact postciliary microtubules and that the oral dikinetids rotated into place during stomatogenesis to give this unusual orientation (see below Division and Morphogenesis ). Both anterior and posterior kinetosomes of the oral dikinetids bear a postciliary ribbon and may also have one or two transverse microtubules (Hiller, 13.4 Oral Structures 265 266 13. Subphylum 2. INTRAMACRONUCLEATA: Class 7. PROSTOMATEA 1993b). The postciliary microtubules appear to be used in several different ways among the pros- tomes. In Holophrya (formerly Prorodon ) species, the oral postciliaries may extend anteriorly sup- porting cortical ridges that cover the walls of the oral opening (Hiller, 1993b). In Balanion , Coleps , and Plagiocampa , oral palps , bearing a toxicyst , are placed internal to the oral dikinetids (e.g., Bardele, 1999; Fauré-Fremiet & André, 1965b). These palps are supported by microtubules, pre- sumably derived from the postciliary ribbon of the anterior kinetosome of the oral dikinetids (Bardele, 1999). In a third group of species (e.g., Balanion , Bursellopsis , Urotricha ), the postciliary microtubules of the posterior kinetosome of the oral dikinetids extend around the perimeter of the oral opening in a counter-clockwise direction as viewed from outside the cell. They may overlap as many as 12 other ribbons and appear to be joined together by intermicrotubule bridges, which may permit slid- ing to dilate or close the oral opening (Hiller, 1991, 1993b). The oral dikinetids typically sit atop two nematodesmata that are triangular to trapezoidal in cross-section. Each nematodesma extends into the cytoplasm and is joined by dense material deeper in the cytopharyngeal basket to the nematodesma arising from a neighboring oral dikinetid (Hiller, 1993b; Lynn, 1985). Since postciliary ribbons sup- port the oral cavity and line the cytopharynx, the cytopharyngeal apparatus of prostomes must now be designated as a cyrtos \u2013 the rhabdos is now only found in the Class LITOSTOMATEA . The oral ridges on the walls of the oral cavity have been observed in all prostomes , including Cryptocaryon (Colorni & Diamant, 1993; Diggles, 1997). These ridges are covered by cortical alveoli down to the level of the cytostome (Hiller, 1993b; Hiller & Bardele, 1988; Huttenlauch, 1987; de Puytorac, 1964). There are typically two sets of microtubules in these ridges, one set composed of more microtubules than the other (Hiller, 1993b; Lynn, 1985). However, these can be reduced to 3 + 2 in Bursellopsis , very reminiscent of the oral ridges in oligohymenophoreans (Hiller, 1991). These microtubules are presumably derived from the postciliary ribbons of the oral dikinetids (Huttenlauch & Bardele, 1987), and ultimately extend to line the cytopharynx where they may function to bring new food vacuole membrane to this region (Hiller, 1993b; Rodrigues de Santa Rosa, 1976). The brosse or brush is considered here as an oral structure. As noted above, its kinetosomes can be within the circumoral dikinetids , opposite a paroral set of oral dikinetids or outside the oral perimeter entirely. In both colepids and prorodontids , the brosse is composed of rows of dikinetids, whose axes are typically oriented perpendicularly to the longitudinal axis of the brosse rows. The anterior (or left) kinetosome bears a typically shorter, clavate-like cilium and may or may not bear a tangential transverse ribbon while the posterior (or right) kinetosome is non-ciliated and bears a postciliary ribbon (Hiller, 1991, 1993b). In species without oral palps , toxicysts are concentrated in the oral region, where they are presumably used to soften prey, like the fila- mentous alga Spirogyra (Leipe, 1989). Similar to the long toxicysts of Dileptus , the toxicysts of Holophrya (formerly Prorodon ) demonstrate a combination of tubule evagination and tele- scoping (Hausmann, 1978). Placus exhibits an unusual fosette or pit at the posterior end of its single brosse row. The brosse row extends into this pit in which there is a dense aggregation of toxicysts (Fryd-Versavel, Iftode, & Dragesco, 1976; Grain et al., 1979). 13.5 Division and Morphogenesis Prostomes typically divide while swimming freely. However, histophagous and parasitic prorodontids, like Holophrya and Cryptocaryon , can form reproduction or division cysts (Fig. 13.1) (Czapik, 1965; Diggles, 1997; Hiller & Bardele, 1988). Even microphagous forms can form temporary division cysts (Tannreuther, 1926). Hiller (1992) has reviewed the merotelokinetal stomatogenesis of prostomes and noted that since proliferation of kinetosomes occurs in a localized region of the cor- tex, this region should be denominated ventral. The oral region becomes prostomatous or apical follow- ing cytokinesis by allometric growth of somatic kineties that \u201cpush\u201d the ventral oral region ante- riorly (Fig. 13.5). The brush or brosse , therefore, cannot be considered a dorsal structure, although Hiller (1992) prefers not to consider it ventral. As viewed from outside the cell, the circumoral ciliature forms by a clockwise migration of the cir- cumoral dikinetids to form a partially or completely closed circle. In Coleps , the circumoral arises from one somatic kinety (Fig. 13.4; Huttenlauch & Bardele, 1987). The number of kineties involved in circumoral dikinetid formation probably increases with increasing cell size : only two in Urotricha (Muñoz, Téllez, & Fernández-Galiano, 1989), three to five in Bursellopsis (Hiller, 1992), and up to six in Holophrya (formerly Prorodon